Prevent SystemStackError when calculating slot shares

Avoids infinitely recursing through sibling slots when all demands are
zero.

Ref #58
Ref #59

lib/refinery/slot.rb

@@ -68,8 +68,13 @@ def edges
     #
     #   * Otherwise, the share cannot be determined automatically.
     #
+    # recursing - Flag indicating if `share` is being called from the `share`
+    #             method of other slot. Prevents infinitely recursing through
+    #             all the slots of a node when demand is zero and the shares
+    #             cannot be calculated.
+    #
     # Returns a Rational or nil.
-    def share
+    def share(recursing = false)
       if (explicit = get(:share)) then return explicit end
 
       slots = @node.slots.public_send(@direction)
@@ -91,7 +96,8 @@ def share
             # Special-case for when a node demand is zero, and contains a slot
             # with "normal" links, and a slot with overflow links.
             set(:share, 0.0)
-          elsif node_demand.zero? && others.all?(&:share)
+          elsif ! recursing && node_demand.zero? &&
+                others.all? { |o| o.share(true) }
             # Opposite of the special case above.
             set(:share, 1.0 - others.sum(&:share))
           elsif ! node_demand.zero?

spec/integration/parent_and_two_children_spec.rb

@@ -267,6 +267,26 @@
       mother.slots.out(:electricity).set(:share, 0.4)
     end
 
+    context 'and the children have zero demand' do
+      #         [M]
+      #    :gas / \ :electricity
+      #   (0) [C] [S] (0)
+      before do
+        mother.slots.out(:gas).set(:share, nil)
+        mother.slots.out(:electricity).set(:share, nil)
+
+        child.set(:demand, 0.0)
+        sibling.set(:demand, 0.0)
+
+        calculate!
+      end
+
+      it 'does not set the slot shares' do
+        expect(mother.slots.out(:gas).share).to be_nil
+        expect(mother.slots.out(:electricity).share).to be_nil
+      end
+    end
+
     context 'and the parent defines demand' do
       #         [M] (50)
       #    :gas / \ :electricity